Metals having a tendency to lose one or more electrons to form cations, i.e., metals with high ionization tendency, are mentioned as metals suitable as negative electrode active materials, one of the fundamental components of a battery. One example thereof is lithium metal. Batteries having lithium metal as the negative electrode active material are, in combination with various positive electrode active materials, such as oxides and sulfides, configured as nonaqueous electrolyte batteries. They have been commercialized and used mainly as power supply for small portable electronic devices.
In recent years, in order to provide small portable electronic devices with improved convenience, attempts have been successfully made to reduce the size, weight, and thickness thereof, while improving the performance. With such advances, batteries used as power supply for these devices are required to be smaller, lighter, and thinner, and also, in particular, to have a higher capacity. Therefore, with respect to the capacity per unit mass (mAh/g) or the capacity per unit volume (mAh/cm3) of a negative electrode active material and a positive electrode active material forming a battery, it can be said that the higher the capacity, the better.
The energy capacity per unit mass of lithium metal (Li) is larger than that of other metals, and it thus is superior to the others. Accordingly, a number of researches have been reported on lithium secondary batteries. However, lithium secondary batteries have safety problems. Further, lithium is limited in resources and is expensive.
In comparison, magnesium is rich in resources and is much less expensive as compared with lithium. Further, magnesium metal has a high energy capacity per unit volume, and also has a higher energy density than lithium metal. Moreover, when used in a battery, high safety is expected. A magnesium battery is thus a battery capable of compensating for the disadvantages of a lithium secondary battery. Against this background, greater importance is being placed on the development of, as a next-generation, high-capacity battery, a nonaqueous electrolyte battery using magnesium metal as the negative electrode active material.
For examples, a magnesium secondary battery capable of 2000 or more charge/discharge cycles is reported in the below-mentioned D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich, E. Levi, “Prototype systems for rechargeable magnesium batteries”, Nature, 407, pp. 724-727 (2000) (pp. 724-726, FIG. 3) (hereinafter hereafter to as Nonpatent Document 1) and JP-T-2003-512704 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application) (pp. 12-19, FIG. 3). In such a battery, magnesium metal is used as the negative electrode active material, and a Chevrel compound CuxMgyMo6S8 (x is 0 to 1, and y is 0 to 2) is used as the positive electrode active material. As the electrolyte, an electrolyte represented by a general formula Mg(ZX1R1mR2n)2 (Z is boron (B) or aluminum (Al), X is chlorine (Cl) or bromine (Br), R1 and R2 are hydrocarbon groups, and l+m+n=4) dissolved in an aprotic solvent tetrahydrofuran (THF) or the like is used.
The Chevrel compound is a host-guest compound having Mo6S8 as the host and Cu2+ and Mg2+ as the guest. As shown in FIG. 7, Mo6S8 exists in the form of a cluster, in which six Mo atoms in the shape of a regular octahedron are surrounded by eight S atoms in the form of a regular hexahedron. A number of such clusters are regularly piled up to form the basic crystal structure. Cu2+ and Mg2+ occupy a position the channel region between two clusters, and are weakly bonded to Mo6S8.
Therefore, Mg2+ can move in the Chevrel compound relatively easily. At the time of discharge, Mg2+ is immediately stored in the Chevrel compound, while at the time of charge, the stored Mg2+ is immediately released. The amount of metal ions stored in the Chevrel compound can be varied over a wide range by the rearrangement of electric charges on Mo and S. Results of X-ray analysis have revealed that between two Mo6S8 clusters, there are six sites A and six sites B capable of storing Mg2+. However, Mg2+ cannot occupy all of the twelve sites simultaneously.
Meanwhile, compounds generally called manganese dioxides encompass a wide variety of oxides with an apparent manganese valence of three to four, as well as those containing, in addition to manganese and oxygen, various cations and neutral molecules, such as water, in their structure. Various compounds with an extremely wide variety of compositions and structures are known. Such manganese dioxides have a manganese redox couple with relatively high electric potential, and thus are capable of forming batteries with high electromotive force. In addition, manganese is rich in resources, inexpensive, and highly safe. For these reasons, manganese dioxides have been studied and developed as positive electrode materials for aqueous solution batteries such as dry cells and also for nonaqueous solvent batteries such as lithium secondary batteries.
As manganese dioxides, various compounds are known having, as constitutional units, octahedrons in which six oxygens are coordinated around manganese. Such octahedrons are linked one another sharing faces, edges, and vertices, providing the octahedral chain structure of the compound. Examples thereof include a compound with a tunnel structure having a quadrangular section whose side has a length of one octahedron (1×1) and a compound with a tunnel structure having a quadrangular section whose side has a length of two octahedrons linked (2×2). Such a tunnel can be used as a channel (pathway) for ion diffusion. Therefore, compounds with a tunnel structure are potential positive electrode active materials, and the use thereof as electrode materials for lithium secondary batteries and the like has been proposed.
In this case, β-manganese dioxide (tetragonal system) has a tunnel dimension of (1×1), and γ/β-manganese dioxide (orthorhombic/tetragonal system) has tunnel dimensions of (1 ×1) and (1×1), which are both relatively small; in contrast, the tunnel dimension of an α-manganese dioxide (monoclinic system) is as relatively large as (2×2). When the tunnel dimension is small, this raises the concern that the crystal structure might be collapsed due to the repetition of storage/release of ions. Therefore, α-manganese dioxide with a large tunnel dimension, for example, is expected to be useful as a positive electrode active material (see the below-mentioned JP-A-7-144918 (claim 1, pp. 2 and 3, and FIG. 1) and JP-A-2003-86179 (claim 2, pp. 2-5, and FIGS. 1 and 3).
JP-A-7-144918 (claim 1, pp. 2 and 3, and FIG. 1) proposes a method for producing a manganese dioxide, according to which an inorganic salt of manganese (II), such as manganese nitrate (II), and a permanganate, such as lithium permanganate, are reacted in a solution acidized by the addition of an inorganic acid, such as sulfuric acid, thereby giving α-manganese dioxide HxMn8O16 having hydrogen ions and manganese ions as main cations.
Further, JP-A-2003-86179 (claim 2, pp. 2-5, FIGS. 1 and 3) proposes a method for producing a manganese dioxide, according to which a sodium compound, such as sodium carbonate, and a manganese compound, such as β-manganese dioxide (mineral name: pyrolusite), are mixed at a molar ratio of Na:Mn=1:5, for example, and the mixture is subjected to heat treatment in an atmosphere with an oxygen partial pressure of 4 atm for 10 hours to reach 600° C., thereby giving α-manganese dioxide represented by a compositional formula Na0.20MnO2 by a dry process without the use of water.
Under the present circumstances, the energy capacity of the magnesium secondary battery reported in Nonpatent Document 1 and JP-T-2003-512704 (pp. 12-19, FIG. 3) is one half that of a lithium ion battery or even lower. As compared with existing batteries, it is difficult to increase the capacity of such a magnesium secondary battery. This is because of the low energy capacity per unit mass of molybdenum sulfide used as the positive electrode active material.
For example, at the time of discharge, provided that the Chevrel compound functions to the full extent and, starting from the state represented by a chemical formula Mo6S8, receives two Mg2+ (formula weight: 24.3) to be brought into a state represented by a chemical formula Mg2Mo6S8, the reception of two Mg2+ with a total formula weight of 48.6 requires Mo6S8 in an amount equivalent to one chemical formula (formula weight: 832.2). That is, the energy capacity per unit mass of the Chevrel compound is only about 1/34 the energy capacity per unit mass of magnesium, and about 34 g of the Chevrel compound is required to extract the energy of 1 g of magnesium.
As can be understood from the above example, in order to take advantage of the characteristics of magnesium metal as a negative electrode active material having a high energy capacity per unit mass, it is necessary to develop a positive electrode active material having a high energy capacity per unit mass.
Meanwhile, manganese dioxides are expected to serve as positive electrode active materials for lithium secondary batteries and the like, but have the problem of low capacity. Further, like the method for producing a manganese dioxide reported in JP-A-2003-86179 (claim 2, pp. 2-5, FIGS. 1 and 3), a method that employs high-temperature, high-pressure synthesis conditions requires special production equipment, which possibly results in high cost.
The invention is aimed to solve the above problems. An object thereof is to provide a high-capacity positive electrode active material capable of sufficiently exploiting the excellent characteristics of magnesium metal or the like as a negative electrode active material, such as high energy capacity; a method for producing the same; and an electrochemical device using the positive electrode active material.